Storage tanks for liquids are widely used commercially. Among the liquid products stored in tanks are gasoline, oil and ammonia, and liquefied gases such as natural gas (methane), propane, ethylene, oxygen and hydrogen. Because some of these and other liquid products are potentially hazardous, many of them are stored in double-shelled tanks or similar structures so that if the inner or primary containment vessel fails, the escaping liquid will be contained by the outer or secondary containment vessel. Often the space between the vessels contains insulation, as see Sattelberg et al U.S. Pat. No. 3,352,443. In some structures, the main secondary containment vessel may be spaced outwardly from a double-walled insulated tank. Laverman U.S. Pat. No. 3,791,164 discloses such a structure in which a vertical concrete wall constitutes a secondary containment vessel around a double-walled insulated tank for storing a cryogenic liquid. The annular space between the concrete wall and the tank is left open to the atmosphere in this embodiment although it can be enclosed by a roof.
Liquid storage tanks of the described types in past years were provided with one or more inlet and outlet pipes for filling and emptying the tanks. These pipes either penetrated one or more of the bottoms or one or more of the lower areas of the side walls. It is the feeling of some engineers and safety experts that it is undesirable to have any penetrations or holes in the bottoms or side walls of such tanks, including the primary and secondary containment vessels or structures because the penetrations compromise the integrity of the shell or wall where they are located. As a result, more and more new tanks have all pipes for filling and emptying the tank penetrate the roof so as to avoid having penetrations in the bottoms and/or lower portions of the tank side walls. Submerged pumps in each tank provide the pumping action to empty the tank.
While running the pipes through the roof solves one problem it creates another. Thus, in the event there is a failure of the primary containment vessel and the escaping liquid is contained by the secondary containment vessel, there are no openings and associated pipes in the tank bottoms or side wall lower portions through which the liquid can be removed from the primary or secondary containment vessels. It is expected that a failure as described would incapacitate the submerged pumps so that liquid could not be removed through the roof. A need accordingly exists for a tank provided with means by which escaped liquid can be removed from the tank primary and secondary containment vessels but which avoids penetrations through the tank bottoms and/or side wall lower portions until such time, if ever, the primary containment vessel fails and liquid escapes to the secondary containment vessel.